Virtually every commodity in the market today is, at one time or another, transported by truck, often in a tractor-trailer rig. For long distance transport, the trailer, or semi-trailer as it is often called, is frequently carried by a rail car.
It is axiomatic that the larger the cargo space for a fully loaded trailer, the lower the per-mile transportation cost for the transported commodities. There are factors, however, which limit trailer size. Various governmental regulations dictate the maximum dimensions of highway trailers. And, of course, if the trailer is to be transported by rail, the dimensions of the standardized rail car dictate the maximum dimensions of the trailer.
In the most common form of piggyback trailer rail transport, each rail car carries two trailers. The standard rail-transportable trailer has a maximum length of approximately 45 feet, a maximum width of approximately 102 inches, and a maximum height above the ground of approximately 162 inches. For the reasons mentioned above, these dimensions are not to be exceeded. It is, thus, impractical to increase cargo space by increasing the overall dimensions of the trailer.
Heretofore, in most rail-transportable trailers, the floor of the cargo space, i.e., the bottom of the trailer body, is several feet off the ground to clear the rear wheels, axial, and suspension of the tractor and the rearwardly disposed wheels and suspension of the trailer.
The introduction of air-bag suspension systems for tractor-trailer rigs has somewhat reduced the distance between the trailer floor and the road surface required to clear the tractor and trailer suspension systems. But the increase in cargo capacity of trailers so equipped has been quite small. U.S. Pat. No. 3,078,104, granted Feb. 19, 1963, to W. G. Chalmers for "INDEPENDENT REAR SUSPENSION FOR SEMI-TRAILERS", and U.S. Pat. No. 3,140,880, granted July 14, 1964, to L. D. Masser for "SUSPENSION FOR AUTOMOTIVE VEHICLES", show representative pneumatic suspension systems.
It has also been proposed to utilize some of the space between the ground and the bottom of the trailer body by equipping the body with a depending support structure between the wheels of the trailer and the wheels of the tractor to gain auxiliary cargo space. U.S. Pat. No. 3,884,502, granted May 20, 1975, to R. F. Wagner for "SEMITRAILER OR THE LIKE", discloses a trailer with this modification. The disadvantage here is that, although the additional support structure adds cargo space, that space is not in communication with the interior of the trailer and, therefore, does not increase the size of the main cargo compartment. The size of commodities that can be carried remains restricted.
Another approach to increasing the cargo capacity of trailers has been the use of the so-called "drop frame" for the trailer body. In this construction, the floor of the cargo compartment is dropped close to the road surface between the suspension apparatus of the tractor and the suspension apparatus of the trailer and also possibly for a short distance to the rear of the trailer suspension. Again, the cargo capacity has been increased, but there are shortcomings to this design. Principal among these is the hump, or rise, in the floor of the cargo compartment required to clear the wheels, axles, and suspension components of the trailer. This makes it impossible to load the trailer with a forklift truck through the rear door of the trailer from a conventional loading dock. The addition of doors to the sides of the trailers, of course, increases the cost of the trailer, and those doors often are not accessible from the loading dock.
U.S. Pat. No. 4,619,578, granted Oct. 28, 1986, to J. H. Routledge for "RETRACTABLE WHEEL SUSPENSION APPARATUS", discloses an interesting arrangement for utilizing the space between opposite wheel assemblies of a container carrier. The tractor disclosed in that patent pulls a horizontally disposed, U-shaped frame which is open at the rear so that the frame can be backed under and around a container. The two legs of the frame are supported by pneumatic suspension systems whereby the wheels of one leg are suspended independently of the wheels on the opposite leg. Because there are no through-axles between the wheels on one frame leg and the wheels on the other leg, the space between the opposite wheels can be occupied by the cargo container. The system disclosed in the Routledge patent is simply too complex and too expensive to be utilized on the common transport trailer.
There continues to be a need, therefore, for trailer suspension apparatus which will permit the floor of the trailer body to be lowered closer to the roadway surface and to be substantially flat from the rear of the trailer through the wheel region of the trailer.